We discuss the power of time-evolving photoionization as a diagnostic tool to measure the electron density of photoionized gas. We apply this technique to a XMM-Newton observation of the ionized absorber in the Seyfert 1 galaxy NGC 4051, and present the first measurements of the its volume density, its distance from the central ionizing source, and so its mass outflow rate. By extrapolating these measurements to high-luminosity, large-black hole mass quasars, we speculate that AGN winds can play important roles both in the AGN-host galaxy and AGN-IGM feedback processes.